The present invention relates to an improvement of a toroidal-type continuously variable transmission used in vehicles such as automobiles.
Conventionally, a toroidal-type continuously variable transmission, as shown, e.g., in FIG. 5, is constructed so that an input side disk 11 and an output side disk 12 are coaxially disposed and opposed to each other inside a housing (not shown).
An input shaft 13 passes through the shaft center of the toroidal transmission section having the input side disk 11 and the output side disk 12. A loading cam 14 is disposed on an end of the input shaft 13. The loading cam 14 transmits the motive power (rotational force) of the input shaft 13 to the input side disk 11 through a plurality of cam rollers 15.
The input side disk 11 and the output side disk 12, having substantially the same shape, are disposed to be symmetrical, and are formed to be substantially semicircular in section as viewed in the axial direction with both opposed surfaces thereof taken into view. A pair of power roller bearings 16 and 17 that transmit motion are disposed to be in contact with the input side disk 11 and the output side disk 12, respectively, within a toroidal cavity formed by the toroidal surfaces of the input side disk 11 and the output side disk 12.
The power roller bearing 16 includes: a power roller 16a that rolls over the toroidal surfaces of the input side disk 11 and the output side disk 12 (the power roller 16a being equivalent to an inner race constituting the power roller bearing 16); an outer race 16b; and a plurality of rolling elements (steel balls) 16c, and that the power roller bearing 17 includes: a power roller 17a that rolls over the toroidal surfaces of the input side disk 11 and the output side disk 12 (the power roller 17a being equivalent to an inner race constituting the power roller bearing 17); an outer race 17b; and a plurality of rolling elements (steel balls) 17c.
That is, the power roller 16a serves also as the inner race that is a component of the power roller bearing 16, and the power roller 17a serves also as the inner race that is a component of the power roller bearing 17.
In this construction the power roller 16a is pivotally attached to a trunnion 20 through a pivot shaft 18, the outer race 16b, and the plurality of rolling elements 16c, and pivotally supported with a pivot O as the center, the pivot O serving as the center of the toroidal surfaces of the input side disk 11 and the output side disk 12.
On the other hand, the power roller 17a is pivotally attached to a trunnion 21 through a pivot shaft 19, the outer race 17b, and the plurality of rolling elements 17c, and pivotally supported with a pivot O as the center, the pivot O serving as the center of the toroidal surfaces of the input side disk 11 and the output disk 12.
The surfaces of contact among the input side disk 11, the output side disk 12, and the power rollers 16a and 17a are supplied with a lubricating oil whose viscous frictional resistance is large, so that the motive power applied to the input side disk 11 is transmitted to the output side disk 12 through the lubricating oil film and the power rollers 16a and 17a.
The input side disk 11 and the output side disk 12 are independent of the input shaft 13 (i.e. are not directly affected by the motive power of the input shaft 13) by virtue of needle bearings 25. An output shaft 24 is attached to the output side disk 12. The output shaft extends in parallel with the input shaft 13 and is rotatably supported by the housing (not shown) through an angular bearing 22.
In this toroidal-type continuously variable transmission the motive power of the input shaft 13 is transmitted to the loading cam 14. When the loading cam 14 is rotated by the transmission of the motive power, this rotational power is transmitted to the input side disk 11 through the cam rollers 15, which in turn causes the input side disk 11 to rotate. The motive power generated by the rotation of the input side disk 11 is transmitted to the output side disk 12 through the power rollers 16a and 17a. The output side disk 12 rotates integrally with the output shaft 24.
At the time of changing the speed, the trunnions 20 and 21 are slightly moved toward the pivot O.
That is, the axial movement of the trunnions 20 and 21 puts the rotating shaft of the power rollers 16a and 17a and the shaft of the input side disk 11 and the output side disk 12 slightly out of intersection. Accordingly, the rotational circumferential speed of the power rollers 16a and 17a loses equilibrium with the rotational circumferential speed of the input side disk 11, and in addition a component of a drive force for rotating the input side disk 11 causes the power rollers 16a and 17a to swing about the pivot O.
As a result, the power rollers 16a, 17a pivot over the curved surfaces of the input side disk 11 and the output side disk 12, thereby changing the speed ratio to either accelerate or decelerate the motor vehicle.
Such a toroidal-type continuously variable transmission is disclosed, e.g., in Examined Japanese Utility Model Publication No. Hei. 2-49411. As conventional examples of the above-mentioned input side disk, output side disk, and power roller bearings, those using AISI52100 (an equivalent of JIS SUJ2 high carbon chromium bearing steel) are known as disclosed in "NASA Technical note, NASA ATN D-8362."
However, the above-mentioned toroidal-type continuously variable transmission produces, when driven, high contact stress (max: 3.5 to 4 GPa) between the input side disk and the power roller bearing and between the output side disk and the power rolling bearing, which in turn causes the power roller bearing to receive a high thrust load. Such a condition causes the conventional rolling bearing to permanently deform portions between the rolling elements and raceways of the inner and outer races.
If the power roller bearing is e.g., a ball bearing, it is known that spin slippage occurs on the power roller bearing upon reception of such a thrust load and that a portion of the power roller bearing subjected to the spin slippage is heated. The heat produced by the spin slippage increases the temperature of the rolling portion of the power roller bearing.
Further, when the power roller bearing is operated at a high speed and subjected to a high thrust load, an increase in the temperature of the rolling portion due to heat becomes so noticeable that the power roller bearing made of the conventional material exhibits the problem of early flaking and fracture.
Furthermore, the rolling element, in particular, which is a component of the power roller bearing, is susceptible to early flaking and fracture because the rolling element is operated under a condition in which heat conductivity is poor, which in turn brings about a significant reduction in the service life of the bearing.